This invention relates to communication systems and more particularly to digital communication systems.
Third generation (3 G) cellular wireless communication systems based on wideband code division multiple access (WCDMA) technology are being deployed all over the world. These systems are standardized by specifications promulgated by the Third Generation Partnership Project (3 GPP). Evolution of WCDMA radio access technology has occurred with the introduction of high-speed downlink packet access (HSDPA) and an enhanced uplink (UL).
FIG. 1 depicts a typical cellular wireless telecommunication system 10. Radio network controllers (RNCs) 12, 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. In general, each RNC directs calls to and from a mobile station (MS), or remote terminal or user equipment (UE), via the appropriate base station(s) (BSs), which communicate with each other through downlink (DL) (i.e., base-to-mobile or forward) and UL (i.e., mobile-to-base or reverse) channels. In FIG. 1, RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26.
Each BS, or Node B in 3 G vocabulary, serves a geographical area that is divided into one or more cell(s). In FIG. 1, BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26, although a sector or other area served by signals from a BS can also be called a cell. In addition, a BS may use more than one antenna to transmit signals to a UE. The BSs are typically coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. The RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
In a communication system such as that depicted in FIG. 1, each BS usually transmits predetermined pilot symbols on the DL physical channel (DPCH) to a UE and on a common pilot channel (CPICH). A UE typically uses the CPICH pilot symbols in deciding which BS to listen to, which is a process called cell selection, and in estimating the impulse response of the radio channel to the BS. It will be recognized that the UE uses the CPICH pilots for channel estimation, rather than the DPCH pilots, due to the CPICH's typically higher signal-to-noise ratio (SNR). The UE uses the DPCH pilots for estimation of the signal-to-interference ratio (SIR), i.e., for DL transmission power control, among other things.
As UEs move with respect to the BSs, and possibly vice versa, on-going connections are maintained through a process of handover, or hand-off, in which as a user moves from one cell to another, the user's connection is handed over from one BS to another. Early cellular systems used hard handovers (HHOs), in which a first cell's BS (covering the cell that the user was leaving) would stop communicating with the user just as a second BS (covering the cell that the user was entering) started communication. Modern cellular systems typically use diversity, or soft, handovers (SHOs), in which a user is connected simultaneously to two or more BSs. In FIG. 1, MSs 28, 30 are shown communicating with plural BSs in diversity handover situations. MS 28 communicates with BSs 16, 18, 20, and MS 30 communicates with BSs 20, 22. A control communication link between the RNCs 12, 14 permits diversity communications to/from the MS 30 via the BSs 20, 22.
New radio transmission technologies are being considered for evolved-3 G and fourth generation (4 G) communication systems, although the structure of and functions carried out in such systems will generally be similar to those of the system depicted in FIG. 1. In particular, orthogonal frequency division multiplexing (OFDM) is under consideration for evolved-3 G and 4 G systems. An OFDM system can adapt its DL transmission parameters not only in the time domain, as in current communication systems, but also in the frequency domain. This can provide higher performance where the DL communication channel varies significantly across the system bandwidth. For example, combined time- and frequency-domain adaptation may yield a capacity gain of a factor two compared to time-domain-only adaptation for a so-called 3 GPP Typical-Urban channel and a system bandwidth of 20 megahertz (MHz).
As described above, cell selection and handover are fundamental functions in cellular communication systems in that these functions determine which cell(s) a remote terminal communicates with. The terms “cell selection” and “handover” are sometimes given distinguishable meanings. For example, “cell selection” can refer to a function in an idle terminal and “handover” can refer to a function in an active terminal. Nevertheless, the term “cell selection” is used in this application to cover both functions for simplicity of explanation.
Cell selection has a number of objectives, which include connecting terminals to the cell(s) that will provide the highest quality of service (QoS), consume the least power, and/or generate the least interference. It is also of interest to make robust cell selections, thereby limiting the number and frequency of cell re-selections.
Cell selection is traditionally based on the signal strength or SNR of candidate cells. For example, U.S. patent application Ser. No. 11/289,001 filed on Nov. 29, 2005, by B. Lindoff for “Cell Selection in High-Speed Downlink Packet Access Communication Systems”, which is incorporated here by reference, describes a cell selection process that also takes into account the delay spread of the communication channel. For a given SNR, different delay spreads yield different qualities of service (e.g., different bit rates), and by taking this into account in the cell selection procedure, improved QoS can be achieved. In that patent application, the path delay profile in a typical WCDMA communication system is described as a useful representation of the delay spread.
It seems unlikely that estimation of the delay spread in an OFDM communication system would be done in the same way as in a WCDMA system. Moreover, the delay spread does not capture all of the variability of the communication channel, which also arises from the mobility of the UE and relay nodes or BSs with respect to one another, and from the correlation properties of signals transmitted from different antennas. Highly correlated antennas, which is to say antennas that produce signals that are highly correlated, yield little diversity gain, and so such antennas result in greater signal variations at receivers, leading to decreased cell selection accuracy. Correlation functions and their use in characterizing communication channels such as those in cellular communication systems are described in J. Proakis, “Digital Communications”, Section 14.1.1, 4th ed., McGraw-Hill (2001).